Study On Microstructures And Properties Of TiC-Reinforced Nb-Based Alloys Fabricated By Laser Solid Forming

The requirements of aerospace structural parts become increasingly harsh.It is very imperative to develop the alloy with high melting point and high performance.The refractory Nb-Ti-Al alloy owned low density and excellent comprehensive performance.Laser solid forming（LSF）technology had a certain space for controlling the microstructures and properties,which could efficiently produce complex parts with excellent performance.In this paper,the TiC-reinforced Nb-Ti-Al alloys were prepared by LSF technology,and the microstructures,mechanical properties and oxidation resistances of these Nb-Ti-Al-TiC alloys were investigated in order to clarify the mechanism of microstructure formation and the mechanism of TiC affecting the properties.The five LSFed Nb-Ti-Al-TiC alloys had excellent metallurgical quality without obvious defects and cracks,and their constitute phases were all Nbss and（Nb,Ti）C.Both columnar Nbss grains and equiaxed Nbss grains existed in Nb-37Ti-10Al-3TiC and Nb-37Ti-10Al-5TiC alloys,while the Nb-37Ti-10Al-8TiC,Nb-37Ti-10Al-10TiC and Nb-37Ti-10Al-12TiC had completely equiaxed Nbss grains.There were different morphology of（Nb,Ti）C in various Nb-Ti-Al-TiC alloys.The addition of TiC in LSF Nb alloy affected the morphology of Nbss grains and the formation mechanism of（Nb,Ti）C phase.The grain sizes of Nbss in Nb-Ti-Al-TiC alloys decreased first and then increased with the increase of TiC content,and Nbss had the smallest size（22.08μm）with 8at.%TiC addition,but the refinement effect on the grains was weakened when the TiC content was greater than 8at.%.The effect of TiC on grain refinement was mainly due to the obstacle of（Nb,Ti）C on Nbss grain growth and the promotion of columnar to equiaxed transition（CET）by TiC.The effect of laser process parameters on the microstructure was obtained using the calculation of CET model and the 3D-Rosenthal model,it was indicated that the fully equiaxed grains were easier to be obtained when the temperature gradient G was less than 10000 K/cm and the solidification rate R was greater than 0.1 cm/s.The yield strength（YS）,tensile strength（UTS）and elongation after break（El）increased significantly with the increment of TiC content and then decreased.Nb-37Ti-10Al-8TiC alloy owned the highest YS and UTS of 730.11±16.30MPa and 754.14±3.84MPa,respectively,and Nb-37Ti-10Al-5TiC alloy had the highest El of 2.96%.Nb-Ti-Al-TiC alloys had low density and high strength compared to other Nb alloys.The improvement of the tensile properties of these alloys due to proper addition of TiC was mainly attributed to the fine grain strengthening effect and the dispersion strengthening of the precipitated（Nb,Ti）C.The fracture toughness of the five Nb-Ti-Al-TiC alloys were 28.64MPa·m^1/2,29.95MPa·m^1/2,34.96MPa·m^1/2,28.93MPa·m^1/2 and 23.33MPa·m^1/2,respectively,and the results showed that the fracture toughness values of the alloys increased and then decreased with the addition of TiC.Theσ_0.2of Nb-Ti-Al-TiC alloys first increased with the raise of TiC addition,reaching a maximum value of 718.03±37.96MPa of Nb-37Ti-10Al-8TiC alloy,then theσ_0.2 decreased with more TiC addition.Meanwhile,a back propagation（BP）neural network models for the strength and fracture toughness of Nb alloy were established in this paper,they all had good prediction accuracy.The Nb-Ti-Al-TiC alloys were oxidized at 600℃/59h,and the oxide scales were still dense and intact after 59h,and the exfoliation was not found.The oxidation products of Nb-Ti-Al-TiC alloys were Nb₂O₅,Ti O₂ and a small amount of Al₂O₃.The surface of the oxidized alloy consisted of the black flat oxide region and white oxide region with cracks,which were derived from the oxidation of Nbss and（Nb,Ti）C phase,respectively.In the later stage of oxidation,there was no significant change for the macro-morphology and micro-morphology of the oxidation scale with the increase of oxidation time,which showed stable oxidation scale was formed of Nb-Ti-Al-TiC alloy.Compared with other typical Nb alloys,the LSFed Nb-Ti-Al-TiC alloy had a lower oxidation rate and excellent oxidation resistance at 600℃ due to the influence of TiC addition on the grain morphology of Nbss and the alloying effect of Al.The oxidation experiments of Nb-Ti-Al-TiC alloys at 800℃/13h were conducted.The surface of Nb-37Ti-10Al-5TiC specimen was still macroscopically dense and intact after 13h,and the folds and cracks could be observed on the oxidation scales of Nb-37Ti-10Al-8TiC,Nb-37Ti-10Al-10TiC and Nb-37Ti-10Al-12TiC.The oxidation products were Nb₂O₅,Ti O₂,Al₂O₃ and Ti Nb₂O₇ phases after 13h oxidation,and Nb-37Ti-10Al-8TiC,Nb-37Ti-10Al-10TiC and Nb-37Ti-10Al-12TiC alloys showed rupture and exfoliation of oxide scale based on microscopic observations.This were mainly related to the volume change of multiple oxides of（Nb,Ti）C phase and the weak of the bonding force between oxide scale/substrate.Combining the results of microstructure and properties of Nb-Ti-Al-TiC alloys,it could be concluded that TiC improved the microstructure by promoting CET transformation and impeding the growth of Nbss grains,and improved the mechanical properties by fine grain strengthening and dispersion strengthening,so that Nb-37Ti-10Al-8TiC obtained fully equiaxed grains and excellent overall performance.It was shown that the appropriate amount of TiC addition is beneficial to improve the overall properties of Nb alloys.Therefore,the investigation on LSFed Nb-Ti-Al-TiC alloys could provide the basis for promoting the wide application of Nb alloys.